De-excitation of Electronically Excited Sodium by Nitrogen

Abstract

A semiquantitative calculation is made of the cross section for the quenching of Na(32P) by molecular nitrogen, as a function of initial kinetic energy and of final vibrational quantum number, υ f , of the nitrogen molecule. The large observed cross section, which is of gas‐kinetic order, can be explained in terms of an intermediate ionic state, involving Na+ and N 2 − (υ = υ − ) . This state is unstable at infinite separation of Na and N2, but because of the Coulomb attraction it becomes stable at collision distances below about 3 Å. As a result of the vibrational structure of both the intermediate and final states, we treat the reaction in terms of a diffusion of the probability flux through a two‐dimensional network of potential‐energy curves parametrized by both the electronic state and also the vibrational quantum numbers υ − and υ f . At each potential‐energy curve crossing we compute the transition matrix element for insertion into a Landau–Zener type of transition probability. The transition matrix element is represented as the product of an electronic interaction function (obtained from a correlation, due to Hasted and Chong, of results obtained from charge‐transfer processes involving multiply charged ions) and a vibrational overlap integral or Franck–Condon factor. Results are also presented on the quenching of Na (4 2 P) by N2, and on the quenching of Na (3 2 P) by CO. All the results have the same general character: The total cross section is of gas‐kinetic order and depends only weakly on kinetic energy. The partial cross sections for excitation of the different final vibrational levels vf show a rather broad distribution, with somewhat more than half the energy of electronic excitation ending up as vibrational excitation.